Without limiting the scope of the invention, its background is described in connection with road surfacing construction and many other parts of a roadway construction. More particularly, the invention describes the addition of polymers to bitumen for use in asphalt products. While asphalt is used mostly in road construction, the scope of the present invention covers other uses of asphalt—for example in roof shingles and commercial roofing.
Asphalt binder may be characterized generally as an organic cementitious material in which the predominant constituents are bitumens as they may occur in nature or as they may be produced as byproducts in petroleum refining operations. Asphalt binder can generally be characterized as a dark brown or black solid or highly viscous liquid, which incorporates a mixture of paraffinic and aromatic hydrocarbons as well as heterocyclic compounds containing Group 15 or 16 elements, such as nitrogen, oxygen or sulfur.
Asphalt binders have many industrial applications involving use as paving or road surfacing materials, waterproofing material, roofing applications, etc. Perhaps the most widespread use of asphalt binder compositions is in road surfacing and paving applications. The asphalt binder may be used alone, such as where it is applied to the surface of an existing paving structure—as tack coat or an emulsion-based product—or it may be used as a hot mix asphalt (HMA) material which is a composition of an asphalt binder (4 to 10% by weight) and aggregates in a compacted format. Other additives are added also, if needed, to the mixture.
Cracking induced by fatigue is considered a primary mode of distress in asphalt pavements. The viscoelastic properties of the asphalt binder determine to a great extent the fatigue performance of asphalt mixes (Bahia et al., “Characterization of Modified Asphalt Binders in SUPERPAVE MIX DESIGN (2001)). The fatigue resistance of binders is currently characterized using the fatigue parameter, G*sin δ. This parameter is determined using a Dynamic Shear Rheometer (DSR) measurements at 1% strain rate, as per AASHTO T315, to ensure that the asphalt binder remains within the linear viscoelastic region.
The asphalt binders can be modified through the use of polymers. Polymers cover a wide range of modifiers, with elastomers (rubbers or elastics) and plastomers (plastics) being the most commonly used methods of modifications. Styrene-butadiene rubber (SBR) and styrene-butadiene-styrene (SBS) are the most frequently elastomers. These polymers are used in industry to reduce rutting and improving the fatigue and thermal cracking resistance. Polymer-modified asphalts or “PMA” function to provide improved wear and longevity characteristics as a paving material which is advantageous for use on major highways—but at the expense of having to overcome a substantially increased viscosity during the production and paving processes.
In addition to many polymers, it is generally considered to be a conventional practice to incorporate crumb rubber modifiers (CRM) into a bitumen material to form asphalt paving or sealing materials. Such scrap or recycled rubber particles are also referred to as ground tire rubber (GTR) or tire rubber crumbs or tire crumbs and can include materials recovered from a variety of tire carcasses, reclaimed tire treads, crumbs obtained from passenger and or truck tires, mining and agricultural tires and the like. For the purposes of this description, the terms “tire crumbs”, “tire rubber crumbs”, “ground tire rubber”, “crumb rubber modifier” are used interchangeably.
The asphalt base material incorporating such tire rubber crumbs and particles can be of any suitable type such as derived from petroleum refining operations and include aliphatic and aromatic hydrocarbons and heterocyclic compounds, including asphaltenes and maltenes of fairly high molecular weight. Aggregate particles as described above, ranging from sand to crushed rock the size of perhaps ¼-½ inch, or larger, can also be incorporated into the asphalt base material. In addition, in many cases, a blend of asphalt binder can be applied as a sealer coat on top of existing road paving. Such sealing coats typically may be a thickness of ⅛-¼ inch and may incorporate aggregate and additive materials, or relatively finely ground aggregate materials can be dispensed on the sealer coat while it is hot, immediately after its application or after allowing the sealer to set for a period of a few hours or days. Yet another procedure for providing an asphalt/aggregate road composition involves depositing a layer of the aggregate material on a substrate surface such as a road bed or the like and thereafter depositing the asphalt binder material on the layer of aggregate to provide a layer of water proofing materials on top of the structure.
The term “tire crumbs” and other similar terms listed above as used herein, can mean but are not limited to any suitable form of rubber for use in preparing a rubber-modified asphalt, such as particles, granules, and/or other particulate forms (e.g., shavings or flakes, fines, beads, powder, or the like), which can be produced and/or processed in any manner (such as via vulcanization, devulcanization, ambient grinding and/or cryogenic grinding or other methods). Moreover, tire rubber crumbs can exist in suitable size prior to formation of the rubber-modified asphalt products.
In the United States, over 300 million used automotive tires are discarded annually after they have been worn-out during their limited service life. These used tires are essentially a variety of vulcanized rubber products. In some cases, they are hauled to a dump because there is very little use for them after they have served their original intended purpose. A limited number of used tires are utilized in building retaining walls, as guards for protecting boats and similar things where resistance to weathering is desirable. Efforts to reclaim scrap vulcanized rubber have primarily included a physical shearing process, which is suitable for a rubber which can be mixed with asphalt, forming asphalt rubber.
During the vulcanization process of rubber, accelerators, promoters, and/or initiators, are used to form large numbers of sulfur crosslinks. After vulcanization, the crosslinked rubber becomes very stable and cannot be easily reformed into other products. Thus, vulcanized rubber products generally cannot be simply melted and recycled into new products. The sulfur crosslinks which are present in used vulcanized rubber, such as tire rubber, are deleterious in a subsequent curing process which uses used vulcanized rubber as a component in a new polymer mixture. Formulations of tire rubber which use more than minor amounts of vulcanized rubber result in a brittle cured end product unsuitable for many uses such as automobile or truck tires.
In light of the foregoing, various techniques for devulcanizing rubber have been developed. For example, in one devulcanization process, vulcanized rubber is placed in an organic solvent to recover various polymerized fractions as taught in Butcher, Jr. et al., U.S. Pat. No. 5,438,078. Platz, U.S. Pat. No. 5,264,640 teaches taking scrap rubber from used tires and regenerating the monomeric chemicals which are subsequently recovered. This method uses gaseous ozone to break down the crosslinked structure of the rubber followed by thermal depolymerization in a reaction chamber. Platz et al., U.S. Pat. No. 5,369,215 teaches a similar process in which used tire material may be depolymerized under elevated temperatures and at a reduced pressure to recover the monomeric compounds. Myers et al., U.S. Patent No. 5,602, 1 6 discloses a process for devulcanizing rubber by desulfurization. comprising the steps of: contacting vulcanized crumb rubber with a solvent and an alkali metal to form a reaction mixture, heating the reaction mixture in the absence of oxygen and with mixing to a temperature sufficient to cause the alkali metal to react with sulfur in the crumb rubber, and maintaining the temperature below that at which thermal cracking of the rubber occurs, thereby devulcanizing the crumb rubber. Hunt et al., U.S. Pat. No. 5,891,926 is directed to a devulcanization process for rubber in which elevated temperatures and pressures are used to partially devulcanize the rubber. Thereafter, a solvent 2-butanol is used to extract the devulcanized rubber from the non-rubber and/or solids component.
Novotny et al., U.S. Pat. No. 4,104,205 discloses a technique for devulcanizing sulfur-vulcanized elastomer containing polar groups which comprises applying a controlled dose of microwave energy of between 915 MHz and 2450 MHz and between 41 and 177 watt-hours per pound in an amount sufficient to sever substantially all carbon-sulfur and sulfur-sulfur bonds and insufficient to sever significant amounts of carbon-carbon bonds. Other patents directed to microwave techniques include Lai et al. U.S. Pat. No. 4,440,488; Hayashi et al., U.S. Pat. No. 4,469,817; Picker. U.S. Pat. No. 4,665,101; and Wicks et al., U.S. Pat. No. 6,420,457. In general, the application of microwave energy results in uneven heating of the elastomer. As such, the degree to which the elastomeric particles are devulcanized vary within the rubber particle, which is typically most evidenced by different surface and interior properties.
Despite the various devulcanization processes known the art, there remains a need to develop improved devulcanization techniques, especially those that are capable of devulcanizing the tire crumbs in a relatively uniform manner and make it suitable as a bitumen additive for subsequent use in asphalt products in a cost-effective method.